A particle beam therapy system has the configuration in which high-energy charged particles which are accelerated by an accelerator such as synchrotron are extracted as a particle beam from an accelerator and the particle beam which is extracted is transported to an irradiation chamber by particle beam transport system comprising vacuum ducts and deflection electromagnets and then the beam is irradiated onto an affected part of a patient.
Relative dose of a particle beam such as proton, carbon ion beam, etc. reaches a peak value at a deep portion from the surface of body, at a position where these particles stop, that is, just prior to a range of the particle beam. The peak value is referred to as Bragg Peak.
According to a method for treating a cancer by particle beam, the Bragg Peak is irradiated onto a tumor which is formed on human's internal organs. In addition to treatment of cancer, the above-mentioned method can be applied to a case in which a deep portion in body is treated. A part to be treated including a tumor is generally referred to as an irradiation target. A position of Bragg Peak (BP) is determined by energy of a particle beam to be irradiated, and the higher energy of a particle beam is, the deeper a position of Bragg Peak (BP) is formed. In performing particle beam therapy, it is necessary to form uniform dose distribution of particle beam in whole of an irradiation target, therefore, in order to give the Bragg Peak (BP) to whole area of an irradiation target; “spread of irradiation volume” of a particle beam is performed.
A device which is referred to as a ridge filter is used as a device to perform spread of irradiation volume in a depth direction (for example, Patent Document 1). A ridge filter is a device for spreading a width of energy of a particle beam. FIG. 12 is a perspective view showing the configuration of a ridge filter, and the ridge filter is a structure comprising triangular prisms having step-wise sides and the triangular prisms are arranged. This structure is manufactured by shaving a rectangular aluminum material so as to form its outside to be step-wise. Structures comprising triangular prisms are arranged so as for a particle beam to pass through in the direction of axis indicated by Z2 in FIG. 12. Energy of a particle beam is absorbed based on the travelling distance of a particle beam. Consequently, by varying a height from the lowest surface of structure of each step, energy after a particle beam passes through a ridge filter can be changed. Further, by changing a width of a step, a number of particles which passes through a part having a predetermined width of the structure can be changed.
After a particle beam which passes through a ridge filter, as a whole, energy width of the particle is spread. By irradiating an affected part with a particle beam whose energy width is spread, Bragg Peak (BP) having a width in a depth direction is formed, therefore an affected part which has a width in a depth direction can be irradiated with a particle beam.
On the other hand, due to fundamental restrictions of slow emission method, a particle beam which is extracted from a circular accelerator such as a synchrotron accelerator has the property which is different depending on a direction in a plane which is perpendicular to a travelling direction of a particle beam, that is, asymmetry property. When a travelling direction of a particle beam is Z-direction, a direction which is perpendicular to Z-direction and is of a beam circular track plane of an accelerator is X-direction, and a direction which is perpendicular to Z-direction and X-direction is Y-direction, a particle beam which is extracted has comparatively small emittance in X-direction and comparatively large emittance in Y-direction. This asymmetry property of emittance is particularly-problematic for a gantry type particle beam irradiation system which has the configuration such that an affected part is irradiated by changing an irradiation direction. Therefore, there is an example in which symmetrization of emittance is performed by arranging a scatterer in a particle beam transportation system. (For example, Patent Document 2). Further, there is so-called a corkscrew type of gantry which has the configuration such that a beam line is twisted in order to miniaturize whole of system (For example, Patent Document 3).